1. Field of the Invention
The present invention relates to a method and apparatus for measuring the position of a phase interface or boundary during crystal growth.
2. Description of the Related Art
The measurement of the position of a phase interface between liquid and solid that grows upward during crystal growth in a container or the like, which means the measurement of the actual crystal height is of considerable significance for the crystal growth process. Control of the crystal growth process is only possible when the position of the phase interface is measured.
Crystal growth according to the VGF method is usually performed in a closed container under vacuum, inert gas or reactive gas.
Access to the crystal growth chamber in the container is not possible without additional special features, especially with high melting materials and/or with materials that have a high vapor pressure. When a window is built into a container there are always various problems. Furthermore optical windows are generally rapidly fogged or coated, so that no, or no reproducible, measurement of the position of the interface is possible.
Several possible methods are described in the literature however for determination of the position or height of a phase interface:
1. Indirect Methods: Doping elements may be built into crystals with predetermined temperature discontinuities or jumps. The temperature dependence and/or growth rate of the segregation or partition coefficient are used for this purpose. The distribution of the doping elements is then tested or checked in the finished crystal. Since these methods are indirect, they cannot be employed for process control.
2. Ultrasonic Methods (e.g. K. Wilke, J. Bohm in “Crystal Growth”, p. 595). The speed of sound in the crystal can be employed for determination of the location of the phase interface. Transmitter and receiver are mounted outside of the crystal growth container, e.g. a graphite vessel. Complete contact must occur at the interface between all materials along the path of the sound wave from the transmitter to the receiver. However gaps may arise in the path between the growth vessel or container and the crystal due to small differences in the thermal expansion coefficients. These differences however can lead to great measurement errors. Because of the strong temperature dependence of the sound propagation speed additional errors arise. Since the temperature and the temperature gradient change continuously during crystal growth, no accurate calibration of the measurement signal is possible. Furthermore the ultrasonic waves, which are indeed mechanical shock waves, can produce imperfections or interference with the growing crystal front.
3. Mechanical Detection of Crystal Height (see D. C. Stockbarger, “Artificial Fluorite”, J. Optical Soc. Amer. 39, pp.731 to 741 (1949)). The height of the crystals in the growth vessel or container is measured at arbitrary intervals with a detection rod or bar made e.g. of tungsten or graphite.
The mechanical motion of the detection bar or rod can however lead to temperature non-uniformities in the melt and to production of undesirable convection in the melt. The contact of the tip or end of the detection bar or rod can produce a mechanical stress in the crystal as a result, which makes the contacted region of the crystal unsuitable for further use. Also subsequent temperature processing cannot eliminate this sort of defect in the crystal.